The continuing drive to communicate more data from/to more users over a finite wireless bandwidth is driving refinements of multicarrier communication regimens such as orthogonal frequency division multiplexing (OFDM, OFDMA) and multicarrier CDMA (MC-CDMA). OFDM, for example, involves transmitting data on multiple frequencies for the duration of a symbol (typically on the order of about 10 to 100 microseconds, or even up to 1 ms in extreme). By using multiple carriers, termed subcarriers, communication is maintained when one or more subcarriers are adversely affected by narrow-band or multi-path interference. A key aspect of OFDM is that the subcarriers are formed by a mathematical transform that makes the individual subcarriers overlap to some extent. In other communication regimens, overlapping carriers cause interference. OFDM purposely overlaps subcarriers to increase data throughput, and avoid cross-interference by creating the subcarriers by a mathematical transform in an orthogonal manner so they do not interfere with each other even though they overlap in frequency. Use of multiple subcarriers enables a very high degree of scalability: various data rates can be adaptively supported by altering the combinations of subcarriers that form a channel. For example, high data rates are enabled by allocating symbols on many subcarriers in frequency over several time-symbols, to create one high-speed channel. The subcarriers are transmitted in parallel, each carrying a portion of the total data being sent.
For OFDM, Discrete Fourier Transform or Fast Fourier Transforms may be used. Other transforms exist as well. These transforms may include cosine or sine transforms, filterbank transforms or bi-orthogonal transforms. The properties of these transforms differ from properties of OFDM, but they may be applied similarly to create a multicarrier transmission. Even blocked transforms or interleaved transforms (IFDMA) may create alike transmission schemes, where a block of symbols is available on a number of frequency bins at a time.
Orthogonality among the various multicarrier sub-channels dovetails nicely with multi-antenna communications (Multiple Input Multiple Output MIMO and its variations Single Input Multiple Output SIMO and Multiple Input Single Output MISO), which enable increased data throughput and expanded multipath diversity. Multi-antenna transmissions enable extremely high peak-data rates that are increasingly desirable as the wireless transmission of high volume data, such as audio and graphical files, becomes more ubiquitous. Some of these data have also real-time requirements as audio, music and video streams. In order to facilitate high data rates among multiple users without increasing bandwidth, multi-antenna transmissions with high symbol rates set out stringent requirements for pilot sequences and pilot structures. Pilot symbols enable accurate channel estimation over wide bandwidth, which is necessary for reliable demodulation and decoding. Applying various different modulations as BPSK, QPSK, 16QAM and 64QAM set even increasing requirements for channel estimation, because of the sensitivity of demodulation to channel estimation accuracy. Further again, multiantenna transmission and reception techniques set high requirements for channel estimation, as reception typically requires independent channel estimation of all transmitted sequences from all antennas. However, pilot symbols themselves occupy bandwidth that would otherwise be used for data, so the amount of bandwidth used by pilot symbols, and the accuracy of the resultant channel estimate, must be balanced against the overhead within the symbol frame structure that those pilot symbols occupy.
UTRA is a universal terrestrial radio access protocol that is a standard for third generation mobile communications specified by 3GPP (third generation partnership project). UTRA is based on a wideband spread spectrum multiple access and hybrid time-division access methods that have been designed for frequency efficiency, mobility, and quality of service requirements. E-UTRA stands for evolved UTRA, which seeks to expand on the basics of UTRA to establish high performance requirements over a wide area coverage from large macro cells to small micro cells, with a large range of mobile velocity from 0 km/h up to 350 km/h for throughput values ranging from very low bit rates up to ˜100 Mbps. This translates to operation over large range of signal-to-noise ratios, operation over one's own (geographic) cell to other (geographic) cell interference ratios, and very different channel coherence characteristics. Such disparate considerations tend to indicate that different solutions are needed to meet differing conditions of signal to noise ratios (SNR), interference, or whatever might be the limiting factor in maximizing data throughput.
Research is ongoing to meet these requirements using multi-antenna techniques. Researchers expect to achieve significant improvement of throughput values by transmit and or receive diversity at low signal-to-noise ratios with interference rejection combining. On the other hand, for high signal-to-noise ratios, in conditions where the practical rank of the channel so allows, multi stream transmissions from several transmit antennas to several receiver antennas are viable. For such multi-antenna configurations, several techniques of transmitting symbol streams or their rotations are well known. Also several ways of channel coding are known.
A problem arises in how to design pilot structures, which allow these versatile multi-antenna transmission techniques without adding excessive overhead and losing the efficiency of the frame structure. In any case, such pilot patterns need to be a-priori known in the frame structure and can not be varied, as the terminals need to trustworthy know, where to find the symbols of the pilot sequences. This shall also be possible for terminals, which do not have any information of the transmission yet i.e. at initial cell search. Therefore, pilot patterns and pilot code sequences are typically fixed and are written to the system specific standards.
Various pilot symbol schemes have been proposed for OFDM systems for E-UTRA. Reference in this regard may be had to “ORTHOGONAL COMMON PILOT CHANNEL AND SCRAMBLING CODE IN EVOLVED UTRA DOWNLINK”, NTT DoCoMo, NEC, Sharp (London, UK, Aug. 29-Sep. 2, 2005); “PILOT SYMBOL STRUCTURE IN EVOLVED UTRA DOWNLINK”, NTT DoCoMo, NEC, Sharp (London, UK, Aug. 29-Sep. 2, 2005); “EUTRA DOWNLINK PILOT REQUIREMENTS AND DESIGN”, Motorola, (London, UK, Aug. 29-Sep. 2, 2005); “INTER-CELL INTERFERENCE MITIGATION USING ORTHOGONAL PILOT AMONG CELLS FOR DOWNLINK OFDM IN EUTRA”, Panasonic (London, UK, Aug. 29-Sep. 2, 2005); and “DRX/DTX IMPACT ON COMMON PILOT CHANNEL IN E-UTRA DL”, Nokia (London, UK, Aug. 29-Sep. 2, 2005). Also, 3GPP TR 25.814, ver. 1.2 “PHYSICAL LAYER ASPECTS FOR EVOLVED UTRA (Release 7)” serves as a background on the structural constraints that the inventors adopted in devising a solution to optimizing a pilot symbol structure.